Ctrl-testing-Clearing the dust from North Downs tunnel

The North Downs Tunnel, also known as the Blue Bell Hill Tunnel, is a

railway Rail transport (also known as train transport) is a means of transport that transfers passengers and goods on wheeled vehicles running on rails, which are incorporated in tracks. In contrast to road transport, where the vehicles run on a pre ...

tunnel A tunnel is an underground passageway, dug through surrounding soil, earth or rock, and enclosed except for the entrance and exit, commonly at each end. A pipeline is not a tunnel, though some recent tunnels have used immersed tube cons ...

that carries

High Speed 1 High Speed 1 (HS1), legally the Channel Tunnel Rail Link (CTRL), is a high-speed railway linking London with the Channel Tunnel. It is part of a line carrying international passenger traffic between the United Kingdom and mainland Europe; ...

through the

North Downs The North Downs are a ridge of chalk hills in south east England that stretch from Farnham in Surrey to the White Cliffs of Dover in Kent. Much of the North Downs comprises two Areas of Outstanding Natural Beauty (AONBs): the Surrey Hills a ...

, at Blue Bell Hill near

Maidstone Maidstone is the largest Town status in the United Kingdom, town in Kent, England, of which it is the county town. Maidstone is historically important and lies 32 miles (51 km) east-south-east of London. The River Medway runs through the c ...

Kent Kent is a county in South East England and one of the home counties. It borders Greater London to the north-west, Surrey to the west and East Sussex to the south-west, and Essex to the north across the estuary of the River Thames; it faces ...

, south-east

England England is a country that is part of the United Kingdom. It shares land borders with Wales to its west and Scotland to its north. The Irish Sea lies northwest and the Celtic Sea to the southwest. It is separated from continental Europe b ...

. The tunnel is 3.2 kilometres (2 mi) long, with an internal diameter of 12 metres (40 ft) and a cross-sectional area (CSA) of 150 square metres, and descends to a depth of 100 m (325 ft) below the chalk hills. Thus, at the time of its completion, the North Downs Tunnel was both the largest (in terms of CSA) and deepest twin-track railway tunnels to have ever been constructed in the UK. Trains using

can reach 300 kilometres per hour (186 mph) whilst in the tunnel.

Construction

The North Downs Tunnel was constructed as one element of the first phase of the Channel Tunnel Rail Link (CTRL), which was subsequently rebranded as

. Its north-west portal, the nearest to

London London is the capital and largest city of England and the United Kingdom, with a population of just under 9 million. It stands on the River Thames in south-east England at the head of a estuary down to the North Sea, and has been a majo ...

, is positioned at the head of the Nashenden Valley while the south-facing portal emerges at the base of Bluebell Hill. At the time of its completion, the tunnel was reportedly regarded as being one of the largest railway tunnels to be constructed in Europe as well as the largest and deepest tunnel in the UK, possessing a length of 3.2km and an excavated cross section of up to 174m (13m diameter). A key reason for the larger than normal cross section was a result of environmental sensitivity having prevented the construction of any conventional pressure relief shafts for the tunnel, thus necessitating alternative means for handling the pressure waves generated by

high speed train High-speed rail (HSR) is a type of rail system that runs significantly faster than traditional rail, using an integrated system of specialised rolling stock and dedicated tracks. While there is no single standard that applies worldwide, lines ...

s. Much of the tunnel's planning and construction was performed by

Arup Group Arup (officially Arup Group Limited) is a British multinational professional services firm headquartered in London which provides design, engineering, architecture, planning, and advisory services across every aspect of the built environment ...

. The tender was subject to considerable revision, which had been attributed with generating a 15% reduction in costs incurred. Responsibility for the design and project management of the tunnel lay with a multidisciplinary team of engineers that specialising in civil engineering, geotechnical, tunneling, environmental, rail operations, and safety engineers. Particularly attention was paid to the North Downs Tunnel's construction, as it had been recognised from the onset of efforts to construct the CTRL that the tunnel was a potentially major risk area that could disrupt the wider programme. It was considered to be the only major tunnel along the first half of the CTRL. The geology that the tunnel was bored through is largely dominated by relatively weak

chalk Chalk is a soft, white, porous, sedimentary carbonate rock. It is a form of limestone composed of the mineral calcite and originally formed deep under the sea by the compression of microscopic plankton that had settled to the sea floor. Chalk ...

beds. Extensive surveys were performed, including a series of boreholes, which helped build a wider understanding of the material in general. Planners decided that, unlike any of the other tunnels that were constructed along the CTRL, the construction of the North Downs Tunnel would use the modern techniques of the

New Austrian Tunnelling method The New Austrian tunneling method (NATM), also known as the sequential excavation method (SEM) or sprayed concrete lining method (SCL), is a method of modern tunnel design and construction employing sophisticated monitoring to optimize various wa ...

(NATM); in accordance with these method, the application of a sprayed concrete lining followed immediately after the mechanical excavation. Throughout the construction process, continuous monitoring was practiced to track both settlement and any ground deformations that occurred. In June 2000, breakthrough, a key milestone in the tunnel's construction, was achieved; at this point, the work was reportedly two months ahead of schedule. One of the changes made during construction was the application of the secondary tunnel lining, composed of

reinforced concrete Reinforced concrete (RC), also called reinforced cement concrete (RCC) and ferroconcrete, is a composite material in which concrete's relatively low tensile strength and ductility are compensated for by the inclusion of reinforcement having hig ...

; after testing revealed the overlying chalk to be stiffer than had been predicted by earlier surveys, it was recognised that less additional strength would be required to meet the required standards, thus the thickness of this secondary lining was reduced. During December 2001, it was announced that construction of the tunnel had been completed; which was five months ahead of schedule while also being £5 million under budget. This outcome, being better than anticipated, has been attributed to a combination of technical innovation, value engineering and an integrated observational approach that was practiced during the programme. Over 100 workers working in shifts 24 hours a day made this possible, despite the site being perceived as a major risk before the project.

References

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External links

Images of the North Downs Tunnel via kentrail.org.uk

Construction details via trid.trb.org
Channel Tunnel Rail transport in Kent Railway tunnels in England Tunnels completed in 2001 Tunnels in Kent 2001 establishments in England